1. Technical Field
The present invention relates to an oscillating apparatus for generating a signal based on a reference signal. In particular, the present invention relates to an oscillating apparatus for generating a signal with use of a PLL circuit.
2. Description of the Related Art
Normally, PLL (Phase Lock Loop) circuits have been widely used as circuits generating a signal with high accuracy. A PLL circuit compares the frequency of a reference signal and the frequency of an oscillation signal by means of a phase comparator, and changes the frequency of the oscillation signal based on the detected phase difference. It is known, however, that the phase comparator cannot detect a minute phase difference in some cases. The range of such undetected phase difference is referred to as a dead zone.
There has been known a method of using a timing shift current (It) in addition to a charge pump current (Ic) in the PLL circuit, as one method of compensating the existence of such a dead zone. FIG. 11 shows a PLL circuit 60 of a conventional charge pump type. The PLL circuit 60 includes a filter circuit 600, an oscillator 610, a frequency divider 615, a phase comparator 620, a reference signal generator 625, a switch circuit 630, a charge current supply 640, and a charge pump current supply 650.
The filter circuit 600 includes a capacitor 605, and outputs a control signal that is based on the amount of charge accumulated in the capacitor 605. The oscillator 610 outputs an oscillation signal of a frequency that is based on the control signal. The frequency divider 615 either divides or multiplies the frequency of the oscillation signal, and outputs the result to the phase comparator 620. The phase comparator 620 compares the oscillation signal with the reference signal outputted from the reference signal generator 625, and detects the phase difference between the oscillation signal and the reference signal. The switch circuit 630, based on the phase difference, controls whether to charge the capacitor 605 according to a timing shift current (It) outputted from the charge current supply 640, or to discharge the capacitor 605 according to a predetermined discharge current (Ic−It).
Currently, we have not recognized any existence of prior art reference, and so the description thereof is omitted.
FIG. 12 shows an effect that the charge current and the charge pump current give to the phase difference in the PLL circuit 60. The reference signal generator 625 generates a reference signal having a period T. In a certain stationary state, the oscillator 610 is outputting an oscillation signal whose phase difference from the reference signal is t. In such a state, when the oscillation signal has a phase difference with respect to the reference signal, the phase comparator 620 outputs a pulse (signal A) corresponding to the phase difference at the time of rising-up of the signal. Otherwise, the phase comparator 620 outputs a signal B. As a result, charge is accumulated in the capacitor 605 when the signal A is outputted, and charge is discharged from the capacitor 605 when the signal B is outputted.
Here, the oscillation signal is in a stationary state, and that its frequency is not changed. Therefore it means that the amount of charge accumulated in the capacitor 605 is undergoing a transition in the vicinity of a certain standard (X). Accordingly, the amount of charge accumulated by output of the signal A is equal to the amount of charge discharged by output of the signal B. Consequently, the following equation (1) is derived.t(Ic−It)=(T−t)It  equation (1)
By modifying this equation, the following equation (2) is derived.tIc=Titt=T×It/Ic  equation (2)
Consequently, the phase difference t is determined based on the ratio between the current values of It and Ic.
There are cases where the sizes of these current values fluctuate according to the fluctuation of the temperature both inside and outside the apparatus, or the fluctuation of the power supply voltage. Conventionally, there have been cases where, when the fluctuation ratios of It and Ic are different from each other, the phase difference t also fluctuates due to the fluctuation.
Additionally, as one index of representing the characteristics of a PLL circuit, a loop band is known which is a frequency band of a jitter component followable by the circuit. This loop band is defined by the characteristics of a PLL circuit, e.g. characteristics of the oscillator 610, the size of the charge pump current, etc. Depending on the application of the PLL circuit, this loop band is desired to be variable. For example, suppose a case where, in the oscillator 610, the characteristics representing the frequency of the oscillation signal per voltage value of an applied voltage fluctuates according to the size of the frequency. In such a case and the like, if the loop band can be varied, it becomes possible to prevent the loop band of the PLL from fluctuating attributable to the output frequency.
For changing the loop band of a PLL circuit, there are two possible methods as follows. The first method is to change the gain of the filter circuit by a switch. The second method is to switch the charge pump current value by a switch. According to these methods, however, switches corresponding in number to the minuteness of the variable resolution of the loop band become necessary. In view of this, a multitude of switches become necessary, which would lead to increase in complication of the circuit or increase in power consumption. A method of adjusting the current value of the charge pump current by means of a DA converter can be also considered. However in such a case, the above-mentioned phase difference t may fluctuate by change in the characteristics of the DA converter in response to the effect from the temperature change, and the like.